1. Field of the Invention
This invention relates generally to radio frequency power amplifiers and, more particularly, the invention relates to methods of managing the signal to noise floor ratio as the operational bandwidth of a digital linearized predistortion amplifier is expanded.
2. Description of the Related Art
Radio frequency power amplifiers are widely used to transmit signals in communications systems. Typically a signal to be transmitted is concentrated around a particular carrier frequency occupying a defined channel. Information is sent in the form of modulation of amplitude, phase, and/or frequency such that the information is represented by energy spread over a band of frequencies around the carrier frequency. In many schemes the carrier itself is not sent since it is not essential to the communication of the information.
When a signal that contains amplitude variations is amplified, it will suffer distortion if the amplifier does not exhibit a linear amplitude and phase transfer characteristic. This means that the output is not linearly proportional to the input. The signal will also suffer distortion if the phase shift, which the amplifier introduces, is not linear over the range of frequencies present in the signal or if the phase shift caused by the amplifier varies with the amplitude of the input signal. The distortion introduced typically includes intermodulation of the components of the input signal. The products of the intermodulation appear within the bandwidth of the signal causing undesirable interference. The products of the intermodulation also extend outside the bandwidth originally occupied by the signal. This can cause interference in adjacent channels and violate transmitter licensing and regulatory spectral emission requirements. Although filtering can be used to remove the unwanted out of band distortion, this is not always practical, especially if the amplifier is required to operate on several different frequencies.
Distortion products that occur at multiples of the carrier frequency can also be produced in a non-linear amplifier, but are relatively easy to remove by filtering. Intermodulation is also a problem when multiple signals are amplified in the same amplifier even if individually they do not have amplitude variations. This is because the combination of the multiple signals produces amplitude variations as the various components beat with each other by adding and subtracting as their phase relationships change.
Amplifiers can introduce some distortion even if they are well designed. Perfect linearity over a wide range of amplitude is difficult to realize in practice. In addition, as any amplifier nears its maximum output power capacity, the output no longer increases as the input increases. At this point the amplifier is not regarded as linear. A typical amplifier becomes significantly non-linear at a small fraction of its maximum output capacity. This means that in order to maintain linearity, the amplifier is often operated at an input and output amplitude which is low enough that the signals to be amplified are in a part of its transfer characteristic that is substantially linear. This is a method of operation, described as xe2x80x9cbacked off,xe2x80x9d in which the amplifier has a low supplied power to transmitted power conversion efficiency. A xe2x80x9cClass Axe2x80x9d amplifier operating in this mode may be linear enough for transmitting a signal cleanly but might typically be only 1% efficient. This wastes power and means that the amplifier has to be large and relatively expensive. In addition, the wasted power is dissipated as heat, which generally must be removed by cooling means.
Communication schemes using signals which have constant amplitude with frequency and phase modulation can use highly non-linear amplifiers. These types of signals are unaffected by the distortion and the amplifiers can be smaller, cooler, more power efficient and less expensive. Modulation of this type is used in conventional radio paging systems, which use CPFSK modulation.
Many of the newer, bandwidth efficient modulation schemes have both amplitude and phase variations. There is also frequently a desire to be able to transmit multiple signals on different channels through a single amplifier. This reduces the number of separate amplifiers required and avoids the need for large and costly high level output signal combining filters, which have undesirable power losses.
There is a need for linear amplifiers which are compact, power efficient and inexpensive. Linearized amplifiers can be made by correcting for the non-linearities of amplifiers using methods such as Cartesian feedback, predistortion, and feedforward correction.
Cartesian feedback is a method in which a monitoring system looks at the output of the amplifier and attempts to alter the input of the amplifier so that it produces the intended output. This is accomplished using a direct feedback loop. The delay in the feedback path can cause the input signal to be modified too slowly to provide effective compensation, especially with signals at higher bandwidths.
The traditional predistortion method attempts to correct for the non-linear transfer characteristic of an amplifier by forming an inverse model of its transfer characteristic. This characteristic is applied to the low level signal at the input of the amplifier in a nonlinear memory-less function to predistort the signal such that the amplified signal appears substantially undistorted. This method is capable of excellent results over a relatively small bandwidth. The non-linear memory-less function is updated to account for variations in the amplifier transfer characteristic and this is done by monitoring the output and periodically updating the correction parameters. The non-linear coefficients of the memory-less function may be changed as often as every sample using the values stored in memory.
Feedforward is a method that derives a signal which represents the inverse of the distortions produced by the amplifier. This can be done by comparing the amplifier input and output to extract a distortion signal. A small linear amplifier is used to amplify the distortion signal. The amplified distortion signal is then subtracted from the main amplifier output. This method gives good results over a relatively wide bandwidth. However, balancing the amplitude and delay of the distortion signal so that it cancels the main amplifier errors exactly is difficult to implement.
Both traditional feedforward and predistortion are widely used in commercial products which can amplify multiple signals and operate over a wide range of amplitudes. Both methods are quite complex and the power efficiencies are still not excellent. Feedforward amplifiers are typically only 5% efficient. The complicated processing requirements add to the cost and the power used and significant cooling capacity is still required to remove waste heat. Predistortion is capable of excellent results, but only over a relatively small bandwidth.
The present invention provides methods of managing the signal to noise floor ratio exhibited in individual subbands as the operational bandwidth of a digital linearized predistortion amplifier is expanded.
In one aspect of the invention, a digital input signal is separated into subbands of lower bandwidth. The digital input signal is preferably a wideband signal that has one or both of the following characteristics: (a) the signal exists at one or more frequencies within an operating bandwidth within a time interval that is the reciprocal of the total information bandwidth; (b) the signal consists of multiple information bearing subcarriers and has a spectral occupancy that exceeds 0.1% of the RF carrier frequency. Each of the digital subband signals, which has a lower power than the digital input signal, is separately converted to an analog subband signal using a separate DAC. The separately converted analog subband signals are combined to form an analog input signal. By separating the digital signal into subbands, separate DACs can be used and the power of the signal to be handled by any one DAC is reduced.
In another aspect of the invention, a digital correction signal is created by taking the difference between a digital predistortion signal and the digital input signal. The digital predistortion signal, which is a signal that is typically passed through a Digital to Analog Converter (DAC) and supplied to a non-linear amplifier, can be created using presently available techniques. In accordance with the invention, however, the digital input signal is removed from the digital predistortion signal to leave only the digital correction signal, which has a much lower power than the digital predistortion signal. The digital correction signal and the digital input signal (or its subbands) are separately converted to analog signals using separate DACs. The converted analog signals are combined by analog summation to form an analog predistorted signal. The analog predistorted signal is passed on to a non-linear amplifier.
The aforementioned aspects of the invention result in the separation of a digital signal into separate signals of lower power. In the prior art, a single DAC has been used to convert a combined signal of much higher power. In accordance with the present invention, however, multiple DACs are employed and each DAC converts a digital signal of a lower power. As a result, the available levels of quantization of each DAC are applied to a lower power signal and the lower power per quantum ratio provides a better signal to noise ratio. Accordingly, substantially the entire dynamic range of each DAC can be used to convert a signal to analog form.
In another aspect of the invention, each analog signal is passed through a separate narrow band reconstruction filter before the analog signals are combined. Each reconstruction filter can be configured specifically for a narrow frequency range. By passing each converted signal through a separate narrow reconstruction filter, a significantly higher signal to wideband noise ratio for the composite signal can be achieved. The use of separate reconstruction filters, however, may introduce relative gain, phase, and delay inconsistencies between the separate signals. These relative inconsistencies are caused by the analog nature of the reconstruction filters, which are preferably configured to handle specific narrow frequency bands.
In order to correct the relative inconsistencies between the separate signals, a digital correction filter is introduced in-line along each subband signal path before the DAC. The correction filters are driven by an Adaptive Control Processing and Compensation Estimator (ACPCE) block, which adaptively generates compensation parameters for the filters based on observations of the digital input signal and the output of the amplifier.